This invention relates to telephone networks and more specifically, to converters for interconnecting portions of such telephone networks.
A telephone network comprises several portions, including central switching equipment and local equipment. The central switching equipment transmits intelligence or voice signals at a variety of frequencies and a ringing signal at a relatively high frequency, for example, 2600 Hertz. Oftentimes, however, the local equipment responds to a ringing signal at a much lower frequency, for example, 20 Hertz. In order for such local equipment and central switching equipment with disparate ringing signal frequencies to operate together, some means must be provided for converting the ringing signal at one frequency to a ringing signal at the other frequency.
Such conversion means must also discriminate between the high frequency ringing signals from the central switching equipment and voice signals which may be in the same range of frequencies. Conventionally, active filters or LC filters are in series between the output from the central switching circuit and define a narrow passband centered on the high frequency ringing signal. However, the selectivity of such bandpass filters depends upon the quality factor (Q) of the filter. LC filters are normally limited to relatively low values of quality factor. If active filters are used, it is necessary to increase amplifier gain to improve selectivity, but increased amplifier gain leads to stability problems.
Further, it is difficult to alter both the bandwidth and the center frequency of these filters. This is a disadvantage in trying to construct a converter, because a given network may use any one of several high frequency ringing signal frequencies.
Therefore, it is object of this invention to provide a telephone conversion circuit with improved ringing signal detection means.
Another object of this invention is to provide an improved ringing signal detection circuit including a digital bandpass filter.
Still another object of this invention is to provide a variable bandwidth, highly selective bandpass filter for operation at a number of frequencies.